The present invention relates generally to the field of spinal implant systems, and particularly systems that employ elongated spinal implants, such as a rod and plates, connected at various locations along the spinal column. More particularly, the invention concerns a connection assembly that provides variable angle and variable height adjustability to the elongated spinal implant relative to a bone fastener engaged to the spine.
Several spinal fixation systems have been developed for use in correcting and stabilizing sections of the spinal column and facilitating spinal fusion. In one such system, a bendable elongated spinal implant, such as a rod, is longitudinally disposed adjacent the vertebral column and then secured to various vertebrae along the length of the column by way of a number of bone fasteners or fixation elements. A variety of bone fasteners can be utilized, such as hooks or bone screws, which are configured to engage specific portions of a vertebra.
An example of one such system is the TSRH(copyright) Spinal System of Sofamor Danek Group, Inc. In this system, various hooks and bone screws are engaged to a spinal rod by way of eyebolts. In early versions of the TSRH(copyright) Spinal System, the vertebral hooks and bone screws were attached to the spinal rod at a fixed orientation, usually projecting perpendicularly below the rod. At the time, the TSRH(copyright) Spinal System presented a significant advance over prior systems in its versatility, strength of fixation, and ease of implantation.
However, one drawback faced by the original TSRH(copyright) Spinal System, as well as the other prevalent fixation systems, was that a surgeon was required to make significant adjustments to the contour of the bendable rod so that the bone fasteners could solidly engage the vertebral bodies. What was needed, then, was a bone fastener that could be connected to the spinal rod at a variable angle. In order to address this need, the TSRH(copyright) Variable Angle Screw was developed, as described in U.S. Pat. No. 5,261,909. This Variable Angle Screw utilized the same TSRH(copyright) eyebolt to achieve a connection to a spinal rod. In addition, the Variable Angle system incorporated a washer that fit over the eyebolt, engaged the spinal rod within a groove in one surface of the washer, and provided a radially splined surface facing the bone fastener. The bone fastener had a complementary splined surface so that the fastener could be situated at variable angular orientations relative to the spinal rod. A nut threaded onto the post of the eyebolt clamped all the components together to complete the assembly.
The Variable Angle Screw system of the ""909 Patent presented a significant advance over prior rod-based implant systems. The system of the ""909 Patent was relatively compact and required a minimal number of parts yet was able to accomplish a solid fixation of the bone fastener to the rod at a wide range of angular orientations. One drawback of the system was that the eyebolt-nut combination required side-tightening of the nut to clamp the system together. This side-tightening aspect required a larger surgical site about the spine so that a wrench could be manipulated. To address this difficulty, a top-tightening assembly was developed as disclosed in U.S. Pat. No. 5,282,801. The clamp assembly depicted in the ""801 Patent replaced the eyebolt and nut with a clamp body having a T-bar against which the head of the variable angle bone fastener was clamped. In addition, while the original TSRH(copyright) System relied upon tightening a nut against the variable angle bone screw, the top-tightening approach of the ""801 Patent utilized a set screw that acted against the spinal rod to push the spinal rod into the interlocking washer, and ultimately against a complementary spline face of the variable angle screw. With this system, the variable angle capability was retained, while a top-tightening feature was added.
With the addition of the top-tightening capability, the more recent TSRH(copyright) Spinal System has provided surgeons with a great deal of flexibility in the placement and orientation of bone fasteners, such as hooks and screws, relative to a spinal rod. The Variable Angle components greatly reduce the need to manipulate and bend the spinal rod to conform to the patient""s anatomy. Even with the great improvements presented by the TSRH(copyright) Spinal System, a certain amount of shaping or contouring of the spinal rod has still been required. Specifically, the rod must be shaped so that at the point of attachment of the bone fastener, the rod is the same distance from the vertebral body as the splined or interdigitating portion of the bone fastener. This vertical or height alignment is necessary so that the variable angle components are properly aligned for accurate connection when the assembly is clamped together. Thus, the spinal surgeon still has to spend a certain amount of time shaping the spinal rods during the surgery so that the fixation system can be properly implanted.
In order to address this difficulty, later systems were developed that provided for a certain degree of vertical adjustability. By vertical or height adjustability, it is meant adjustment along the length of the bone fastener. Adjustment in this dimension allows the rod to be situated at varying distances from the spine, or oriented with a pre-set contour regardless of the location of the fastener.
An adaptation of the original variable angle screw concept of the ""909 Patent is presented in U.S. Pat. No. 5,611,800. This system retained the yoke configuration of the bone screw in the ""909 Patent, but added a multifaceted connecting feature on both surfaces of the yoke. While the ""800 Patent system added height adjustability, it did so at the cost of a more complicated connector structure with four specially machined interdigitating surfaces.
Another approach has been suggested in U.S. Pat. No. 5,643,263. The connection assembly in the ""263 Patent uses a Schanz-type bone screw rather than the yoke bone screw of the ""909 Patent.
Thus, the screw described in the ""263 Patent includes an elongated smooth shank portion. The connection assembly also adds a second washer disposed between the original washer and the smooth shank of the bone screw. The interdigitating feature exists between the contacting faces of the adjacent washers. The variable height is accomplished by a groove provided in the opposite surface of the additional washer that allows the connection assembly to slide along the shank of the bone screw until it is finally clamped together by a set screw.
While the connection assembly shown in the ""263 Patent goes a step further toward an easy-to-implant variable angle, variable height assembly, it too has left some room for improvement. For example, the connector assembly of the ""263 Patent requires an additional washer that adds to the number of components that must be manipulated by the surgeon. In addition, the added washer increases the lateral profile of the implant assembly. In certain regions of the spine, such, as the thoracic and cervical regions, there is limited space transverse to the vertebral bodies. An optimal spinal implant system that has universal applicability should have as small a lateral profile as possible.
Yet another approach for achieving variable angular orientation of the spinal implant assembly has been suggested in U.S. Pat. No. 5,947,967. The connection assembly shown in ""967 includes a body defining a tapered outer surface and an internal tapered washer fitting over the tapered outer surface of the body. The washer includes a slot to receive the bone screw which can lock the screw in position by frictional engagement with the tapered outer surface of the body. Although the connection assembly in ""967 allows the bone screw to adopt variable angular orientations relative to the rod, it does not provide easy access for readjustibility once the connection assembly is fixed to a particular position.
A need has remained for a spinal implant system that not only provides a reliable connection between a bone fastener and an elongated spinal implant or a rod that spans along the spine, while permitting variable angular and height between the two components, but also provides an easy access for reorientation and readjustment of the components. A need has also remained for a spinal implant system that has reduced bulk or prominence to avoid other spinal features and to be undetectable along the patient""s back.
In order to address the unresolved detriments of prior implant systems, the present invention contemplates a variable angle/variable height connection assembly for a spinal implant system. In one embodiment of the invention, a connection assembly includes a first member or body that defines an opening through which an elongated spinal implant, such as a spinal rod, can extend. A second member is provided that includes a second opening through which extends an elongated stem or shank of a bone fastener, such as a bone screw or hook. In some embodiments, the two members are attached by a connecting means that allows the members to pivot relative to each other about a connection axis. This connecting means thus provides a mechanism for variable angular orientations of the bone fastener relative to the elongated spinal implant. The second opening in the second member adds the height adjustment capability.
In another aspect of the invention, the two openings are substantially perpendicular and overlapping. The bone fastener opening has a width that affords variable angular orientations of the bone fastener. In certain embodiments, a wall of the bone fastener opening opposite the spinal implant opening can define an engaging surface. The stem of the bone fastener can be cylindrical or can have substantially flat surfaces configured with projecting ribs for interlocking with the engaging surface. A set screw can be used as a means to urge the spinal implant to press against the bone fastener and the bone fastener against the engaging surface.
In another embodiment of the invention, a connection assembly includes a body defining a first opening sized to receive the elongated spinal implant therethrough and a second opening sized to allow variable angular placement of the bone fastener therethrough. The first opening is elongated along a first axis, and the second opening is elongated along a second axis substantially perpendicular to the first axis. The second opening overlaps the first opening to allow direct contact between the elongated spinal implant and the elongated stem of the bone fastener therein. The second opening is configured to include a plug slot. The plug slot includes a wall opposite the first opening, and the wall defines an engaging surface thereon. The body further includes a first bore intersecting the first opening at an opposite end of the wall.
The connection assembly further includes a plug that can fit snugly within the plug slot. The plug has a mating surface matable to the engagement surface of the wall, and a second surface defining a groove for contacting the elongated stem of the bone fastener. The engagement surface of the wall preferably includes a plurality of radiating splines and the mating surface of the plug includes a plurality of complementary radiating splines. Alternatively, the engagement surface of the wall can include a raised pattern or knurling, and the mating surface of the plug includes a complementary raised pattern or knurling.
Further, the connection assembly can include means extending through the first bore for urging the elongated spinal implant against the elongated stem of the bone fastener and simultaneously urging the elongated stem of the bone fastener against the engagement surface of the wall. When the urging means is loosened, the plug can be rotated so that the groove can receive the elongated stem of the bone fastener at a desired height and angular orientation. Since the plug is accessible from the top side of the connection assembly, it is easy for the surgeon to manipulate or reposition the connection assembly by simply rotating the plug. When the urging means is tightened, the connection plug is pressed against the wall and the spinal assembly is fixed to in final position on the spine.
In an alternative embodiment, a connection assembly includes a body having all the features in the previous embodiment described immediately above. In addition, the body further defines a second bore intersecting the second opening at the center of the wall for receiving a securing means for fastening the plug to the plug slot. In this embodiment, the plug further defines a center bore alignable to the second bore of the body when it is inserted in the plug slot. This embodiment of the connection assembly also includes a securing means defining a securing member extendable through the second bore of the body and the center bore of the plug in the plug slot. The securing member can be a second set screw or a rivet, while both the second bore and the center bore can be threaded to receive the second set screw. The securing member can optionally be loosened or tightened after the assembly is fixed in a particular position on the spine. When the securing member is loosened, the plug can be rotated for angular adjustment for receiving the elongated stem of the bone fastener. When the securing member is tightened, the plug is secured against the engagement wall of the body. In this way, the angular orientation of the spinal implant assembly can be adjusted or readjusted without having to disturb the whole spinal implant system.
In this alternative embodiment, the connection assembly also includes means for urging the elongated spinal implant against the elongated stem of the bone fastener. The urging means includes an urging member extendable through the first bore to contact a spinal implant. The urging member can be a set screw, and the first bore is threaded to receive the set screw. The set screw includes an engagement tip for contacting the spinal implant within the first opening. The set screw can be threaded into the first bore, and as the set screw is driven further into first bore, the engagement tip urges the spinal rod toward the stem of the bone fastener. Continued tightening the set screw increases the clamping force between the spinal implant, the stem of the bone fastener and the plug that has been secured against the engagement wall of the body. Optionally, the securing member only secures the plug inside the plug slot while the plug is rotatable about the center bore axis. Only when the urging member is tightened, the clamping force between the spinal implant, the stem of the bone fastener and the plug presses the mating surface of the plug against the engagement surface of the wall.
In another feature of the present invention, the second opening includes a first side wall and a second side wall, the side walls are raised and converging toward each other leaving a gap sufficient to receive the bone fastener therethrough. Thus, in this feature, the gap essentially acts as a fulcrum about which the body can be pivoted relative to the elongated stem of the fastener to properly orient the connection assembly.
In yet another feature of this invention, a spinal implant assembly, includes a connection assembly as described above, and a bone fastener having an end being an elongated stem and an opposite end being a screw or a hook. The elongated stem in a preferred embodiment is cylindrical and smooth, and sized to fit the groove on the second surface of the plug.
It is one object of the present invention to provide a spinal implant system for variable angle/variable height adjustment. Another objective is to provide spinal implant system components that can be easily clamped together with reduced bulk or prominence to avoid anatomical features and to insure a solid fixation of the instrumented portion of the spine.
Significant benefits are achieved by features of the invention that permit angular adjustment of the spinal implant assembly while most components have been fixed in place. These and other objects and benefits of the invention will be made clear upon consideration of the following written description and accompanying figures.